FRANKFURT - Germany's highest court has declared unlawful a feature that encourages Facebook users to market the social media network to their contacts, confirming the rulings of two lower courts.

A panel of the Federal Court of Justice ruled that Facebook's "friend finder" promotional feature constituted advertising harassment in a case that was filed in 2010 by the Federation of German Consumer Organisations (VZBV).

The Facebook feature invites users to grant it permission to vacuum up the e-mail addresses of friends or contacts in the user's address book, which in turn allows the social network to send an invitation to non-Facebook users to join the service.

The court concluded this was a deceptive marketing practice, confirming decisions by two lower courts in Berlin in 2012 and 2014, which had found that Facebook had violated German laws on data protection and unfair trade practices.

The federal court also said Facebook had not adequately informed members about how it was using their contacts' data.

In a statement, a spokeswoman for Facebook in Germany said it was waiting to receive the formal decision and would study the findings "to assess any impact on our services".

The VZBV welcomed the ruling and said in a statement that it will have implications for other services in Germany which use similar forms of advertising.

"What the judgment means exactly for the current Friends Finder, we now have to find out," said Klaus Mueller, head of the VZBV.

"In addition to Facebook, other services use this form of advertising to attract new users. They must now probably rethink," Mueller added.